1. Field of the Invention
The present invention relates to an optical apparatus and, more particularly, to an optical apparatus wherein circularly polarized beams, having opposite directions and having variations in phase caused by an object to be measured, are superposed on each other, so as to convert variations in physical characteristics such as position, rotation angle, and density of an object to be measured, into a linearly polarized beam, and a rotation amount and rotation direction of the linearly polarized beam are detected so as to detect the variation amounts and directions of the physical characteristics of the object.
2. Related Background Art
Several conventional apparatuses have been provided, each wherein two diffracted beams output from an object to be measured, or one diffracted beam and a reference beam, are superposed on each other, upon incidence of a laser beam on a diffraction grating formed on the object, to form a linearly polarized beam rotating in accordance with a phase difference between the above pair of beams, and a rotation amount and rotation direction of the linearly polarized beams are detected, to thereby detect variations in physical characteristics such as position and a rotation angle of the object. Of these apparatuses, apparatuses for converting a displacement of an object into a change in brightness of interference fringes and detecting a direction of movement of the object are disclosed in U.S. Pat. Application Ser. Nos. 880,207, 196,850 002,229, 002,228, 256,432, for example, of the same assignee.
In some of the apparatuses disclosed in the above-noted U.S. patent applications, two beams output from a diffraction grating formed on the object and to be superposed on each other are converted into linearly polarized beams whose polarization planes are perpendicular to each other. The two beams pass through a phase plate (.lambda./4 plate) having anisotropic properties in directions inclined by 45.degree. with respect to the polarizing directions of the two beams, so that these two beams are converted into circularly polarized beams which are rotated in opposite directions. These circularly polarized beams are superposed on each other to obtain a single linear polarized beam. At this time, a polarization direction of the linearly polarized beam is rotated in accordance with a difference between phases of the above two beams which are superposed first. When this linearly polarized beam passes through a polarizing plate having a polarization axis in an appropriate direction and is received by a photoelectric transducer element, a signal having a sinusoidally changing intensity can be output from the photoelectric transducer element.
The phase of an output signal from the photoelectric transducer element can be arbitrarily changed by appropriately selecting the direction of the polarization axis of the polarizing plate.
For example, when the superposed beam is split into beams, and these split beams are detected through polarizing plates having polarization axes 45.degree. out of phase, the phases of the resultant signals are shifted from each other by 90.degree.. A beam splitter called a half mirror is conventionally available to split the beam into a reflected beam and a transmitted beam. A metal or dielectric material is deposited on a beam splitting surface of the beam splitter on the order of a wavelength. The amounts of reflected and transmitting beams can be adjusted by the type, thickness, and structure of the deposited material. For example, when an amount of a transmitted beam, an amount of a reflected beam, and polarized components suffixed to the transmitted and reflected beams are defined as T, R, P, and S, respectively, the beam splitter satisfies the following equations: EQU T.sub.P =.alpha.R.sub.P EQU T.sub.S =.alpha.R.sub.S
If .alpha.=1, then a ratio of the amount of transmitted beam to the amount of reflected beam is always 1.
It is difficult to cancel a phase difference between P- and S-polarized components of light reflected by the beam splitting surface with respect to the beam splitting surface. The phase difference often varies depending on the wavelength of the incident light and on the incident angle of the light incident on the beam splitting surface.
In a nonpolarization beam splitter having a 1:1 ratio of the amounts of transmitted and reflected beams at 780 nm, the spectral characteristics of the amounts of the transmitted and reflected beams are shown in FIG. 1. The spectral characteristics of the phases of the transmitted and reflected beams are shown in FIG. 2.
When a phase difference occurs between the P- and S-polarized components and when a beam which is incident on the beam splitter is a linearly polarized beam in a direction inclined at 45.degree. from the beam splitting surface, this beam is polarized into an elliptically polarized beam. As a result, when one of the beams (from the nonpolarization beam splitter) which has a larger phase difference between the P- and S-polarized components is observed through a polarizing plate, a loss in the light amount occurs even if the direction of the linearly polarized beam incident on the beam splitter is aligned with the polarization direction of the polarizing plate. On the other hand, even when the direction of the linearly polarized beam is perpendicular to the polarization direction of the polarizing plate, a beam can escape from the polarizing plate and therefore, a contrast level of an output signal, which is changed upon rotation of the polarization plane of the linearly polarized line incident on the beam splitter, is lowered. In the worst case, when the phase difference between the P- and S-polarized components is 90.degree., a beam reflected by the beam splitter becomes a circularly polarized beam. Therefore, the level of the signal photoelectrically converted through the polarizing plate no longer changes.
The phase difference between the P- and S-polarized components is sensitively changed depending on a variation in wavelength of a light source and on an error of an incident angle. For example, if a semiconductor laser is used as a light source, an oscillation wavelength varies on an order of about 10 nm due to variations in the center frequency of the individual lasers and due to changes in temperature. Therefore, it is very difficult to increase a contrast level of the output signal.
As shown in FIG. 2, the above phenomenon typically occurs in a beam reflected by the beam splitting surface of the nonpolarization beam splitter.
When an element such as a total reflection surface which causes a phase difference between P- and S-polarized components is inserted in front of or behind the beam splitter, i.e., in a path for transmitting the rotating linearly polarized beam, the linear polarized beam incident on the reflecting surface at 45.degree. is polarized into an elliptically polarized beam, thus creating the above-noted problem.
In an optical encoder required to detect a displacement direction, two output signals serve as A-and B-phase signals. In this case, the amplitude component of only the A-phase signal obtained by using the reflected beam of the two beams obtained from the nonpolarization beam splitter is changed in response to a variation in ambient temperature, thereby degrading measurement precision.